President Oleksandr Turchynov described the attack to parliament in Kiev, saying the rebels used a portable air defense missile to take down the helicopter. Gen. Serhiy Kulchytskiy, who was in charge of combat training for Ukraine's National Guards, was among those killed, according to Interfax news agency and the BBC.

An Associated Press reporter saw the helicopter go down amid a trail of black smoke. Gunshots were heard in Slovyansk near the crash site and a Ukrainian air force jet was seen circling above. It was too dangerous to visit the site itself.

Meanwhile, Vyacheslav Ponomarev, Slovyansk's self-proclaimed mayor, told CNN that his group of pro-Russian rebels has "detained" four observers from Europe's Organization for Security and Cooperation.

"Our militants got them," Ponomarev said. "They were detained because they didn't respect my request. I asked them not to leave Donetsk (city). They decided they were smarter and could come here."

A spokesman from the Ukrainian Foreign Ministry confirmed to CNN that "negotiations for their release are in process."

Earlier in the week, 50 pro-Russian militia members were reportedly killed by the Ukrainian military in Donetsk, just 70 miles south of Slovyansk.